The advantageous mechanical properties, dimensional stability and optical properties of polyester films are well-known and are exploited in many areas of technology. Many adaptations of polyester films have been proposed in order to tailor the properties of a polyester film to a particular application. However, it is often found that an adaptation which improves the performance of a polyester film in one respect may be detrimental to the performance of the film in another respect. For instance, a modification to improve the optical properties of a film may have a detrimental effect on the mechanical properties of the film. Accordingly, it is often difficult to obtain polyester films having a suitable combination of desirable properties.
In order to improve the performance of polyester films, it is known to provide multi-layer films having complementary properties so as to form a composite structure. In some cases, multi-layer films may comprise two or more films of the same type in order to improve the mechanical strength of the film or to intensify other functional properties of the film. Alternatively, multi-layer films may comprise two or more films of different types, thereby enabling the properties of the different polyester films to be realised simultaneously. For example, multi-layer polyester films have been used to improve the handling properties of functional polyester films by disposing one or more films having desirable functional properties onto a base film having desirable mechanical properties. Laminated polyester films may suitably be prepared by co-extrusion, coating or lamination techniques
However, a disadvantage associated with multi-layer polyester films is that adhesion between layers may be unsatisfactory, which is a particular problem for multi-layer films containing two or more layers of different types. Manipulation of the film, for instance by winding or bending, and/or exposure of the film to moisture or other environmental conditions can lead to delamination of the film layers and to the loss of performance.
The use of elastomeric adhesives in an interposed layer to improve adhesion between the layers of multi-layer polymeric films has been proposed, for instance by EP-A-2039507. In addition, the use of intermediate “tie-layers” in which the intermediate layer comprises components present in each of its adjacent layers as also been shown to improve the delamination resistance of a multi-layer film comprising dissimilar layers. However, such techniques increase the complexity of the film manufacturing process and the intermediate layer can interfere with the mechanical and/or optical properties of the laminated film.